Human congenital cytomegalovirus infection causes extensive damage to many organs, including brain, in about 4,000 children in the United States each year, and is the most frequently recognized viral cause of congenital malformation and mental retardation. An animal model of congenital cytomegalovirus infection might yield information applicable to the human disease. A model system employing two strains of guinea pig cytomegalovirus (GPCMV) has been initiated. One strain seems more virulent than the other; however, both strains have been demonstrated to infect and cross the guinea pig placenta most commonly in the second trimester period. In many cases this fetal infection seems to be inapparent, as it probably is commonly in man; however, in some cases fetal pathology has been noted. Numerous fetal tissues have yielded virus. Occasionally the pregnancy has been aborted following exposure to GPCMV. These preliminary observations suggest that the guinea pig infected with strain specific cytomegalovirus may be an appropriate model for studying many aspects of human CMV infection .. especially as it relates to pregnancy.